The following discussion of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims of this specification
An ICP-MS typically employs an inductively coupled argon plasma (ICP) as an ionisation source and a mass-analyser to separate and measure analyte ions formed in that source. Normally, a sample for analysis is first taken into solution and this solution is pumped into a nebuliser to generate a sample aerosol. The sample aerosol passes into the ICP, where it is atomised and ionised. The plasma is at a relatively high pressure (typically but not necessarily atmospheric pressure, 760 Torr). The resulting ions are then transferred from the plasma via a differentially-pumped interface to a mass-analyser operating at a very low pressure (typically <10−5 Torr). Typically, ions from the plasma enter a first orifice at the tip of a conical body (often called the sampling cone) and then pass through a second orifice, coaxial with the first orifice, at the tip of a second conical body (often called the skimmer cone). The space between the two orifices (a first vacuum chamber) is maintained at a low pressure (1–10 Torr).
The skimmer cone orifice opens into a second vacuum chamber, where the pressure is maintained at around 10−3–10−4 Torr. The ions are extracted from the plasma emerging from the second orifice and focused by ion optics into a mass-analyser, which is located in a third vacuum chamber where the pressure is maintained at 10−5–10−6 Torr.
The mass-analyser separates the ions based on their mass-to-charge ratio, and the separated ions are detected by an ion detection system. The efficiency of the ion extraction/transfer process from the downstream side of the skimmer cone orifice to the ion detector is typically 0.2% or less [1, p. 798].
Ion beam extraction and acceleration towards the mass-analyser typically involve the use of an electrostatic extraction electrode or series of electrodes (hereinafter sometimes alternatively termed “lens”) located downstream of the skimmer cone. To reduce losses, extraction lenses are designed to promote unrestricted pumping from the region immediately downstream of the skimmer cone, where the ion beam is extracted from the plasma. This is to reduce gas molecules in this region, as it is recognised in the art that acceleration of ions through a background gas can lead to losses of ions by scattering, as ions collide with molecules of the background gas. In order to minimise such loses an aerodynamically shaped conical extraction lens may be used as a part of the interface design [2]. This arrangement allows effective removal of gas molecules though the space around the extraction electrode or lens and provides minimum disturbance of the gas flow. Another approach to avoiding disturbance of the gas flow by the extraction electrode or lens and to ensuring adequate pumping efficiency behind the skimmer cone is to place the extraction lens away from the skimmer cone. Yet another approach is to make the extraction lens from coarse mesh grid.
It is known that ICP-MS measurements can be subject to spectroscopic interferences. For example, polyatomic ions, such as ArO+, Ar2+, OCl+ overlap with major isotopes of Fe+, Se+, and V+ respectively, which makes it very difficult to produce reliable analytical results for trace levels of these elements. Other spectroscopic interferences in ICP-MS arise from metal oxide ions. The extent to which such oxide ions are present is monitored by measuring the ratio of cerium oxide ion (CeO+) to cerium ion (Ce+) in the mass spectrum of a sample containing a specified known concentration of cerium. This test is used because cerium oxide ion is the most stable of the common oxide ions. Still other spectroscopic interferences in ICP-MS arise from multicharged metal ions. The extent to which such multicharged ions are present is monitored by measuring the ratio of doubly-charged barium ion (Ba++) to singly-charged barium ion (Ba+) in the mass spectrum of a sample containing a specified known concentration of barium. This test is used because doubly-charged barium is one of the most readily formed of the common multicharged ions. An ICP-MS system that shows simultaneously low values for the CeO+/Ce+ ratio and the Ba++/Ba+ ratio is advantageous because spectroscopic interferences are thereby kept low.
It is known that the effects of some polyatomic ions in ICP-MS can be greatly improved by employing various collision cell techniques [3]. A gas introduced into a multipole ion guide between the interface and the mass-analyser helps to reduce the population of some polyatomic species in the ion beam before the ions enter the mass-analyser. However, this technique is complex and relatively expensive.
U.S. Pat. No. 5,767,512 entitled “Method for Reduction of Selected Ion Intensities in Confined Ion Beams” [4] discloses a method for producing an ion beam having an increased ratio of analyte ions to carrier gas ions by introducing an additional reagent gas downstream of the skimmer cone, thus inducing selective collisional charge transfer.
U.S. Pat. No. 6,265,717 entitled “Inductively Coupled Plasma Mass-spectrometer and Method” [5] describes an ICP-MS interface with a controller for increasing the pressure in the interface (that is, in the enclosure between the sampling and skimmer orifices). This promotes collisions that selectively remove interfering ions. Alternatively, according to the same patent, the local pressure in the interface can be modified by changing the design of the sampling and/or skimmer cone. For example, the sampling cone is modified to give a narrower apex inside the tip. Ions extracted into this narrow apex undergo more collisions because the expansion of the ion beam is restricted.